Using a sediment core from offshore the Murray River, Australian National University researchers have ruled out climate change causing giant kangaroos, three metre-tall flightless birds and the Tasmanian tiger to disappear from the mainland about 50,000 years ago.

Their research concluded that, with these beasts no longer grazing the land, plants flourished – only to fuel huge fires.

Professor Patrick De Deckker from the ANU Research School of Earth Sciences said what was behind the animals' disappearance had caused much debate.

"The extinction happened around the same time as humans moved into the area, which also coincided with a change in the type of plant food available to megafauna," Professor De Deckker said.

"These events have led to several theories of the cause of extinction, including climate change. But the timing of these events was uncertain. We didn't know which one happened first, so we couldn't begin to understand what could have caused the extinction."

Professor De Deckker and his ANU colleagues, in collaboration with scientists from the Netherlands, analysed a sediment core taken from the seabed off the coast of Kangaroo Island in the offshore canyons of the Murray River.

More than 32 metres long, the core was taken in 2003 and has been the subject of several studies.

The Murray and Darling rivers and their tributaries drain a large basin area, so the core reflects changes from a substantial part of south-east Australia, researchers say in a study published in the journal Nature Geoscience.

"Sediment cores provide a record of the past," Professor De Deckker said. "From the core we were able to reconstruct sea-surface temperature over the past 135,000 years, as well as variations in the type of vegetation in the Murray Basin, allowing us to piece together the order of events."

The team found that sea-surface temperature varied by only 3degrees at the time of the extinction – a minor variation compared with other times in the record – indicating that the extinction did not occur during major climate change.

The core also revealed a shift in vegetation type immediately after the megafaunal extinction.

"Before and during the extinction period, 70 per cent of the vegetation was typical of northern Australia today. Immediately after the extinction, this value dropped to 35 per cent," Professor De Deckker said.

"Some people have suggested that this dramatic change in food sources might have been the cause of the megafaunal extinction, but we've shown that this was in fact a result of the extinction. Our idea is that with fewer herbivores around to eat them, substantial fuel remained in the landscape, which eventually led to massive fires.

"Our work unveiled the presence of a compound in the core that is produced as a result of plant material burning. This compound appeared after the megafaunal extinction and lasted some 3000 years."

Professor Tim Flannery predicted this finding two decades ago.

"Professor Flannery suggested that the abrupt extinction of the herbivorous megafauna meant shrubs grew unchecked, increasing the amount of flammable material," Professor De Deckker said. "This explanation has caused considerable controversy, but is now supported by our evidence.

"Management of national parks until recently tried to prevent bushfires. This research shows if the browsers are missing, you end up with a lot of fuel and then you have massive fires.

"I think you can see how fragile the Australian environment is and if you change the equation somewhere, things can tip in a very different way."